Heat pumps are commonly used for heating and cooling buildings, and particularly residential buildings and homes. These heating systems generally are required to provide heating and cooling over a wide range of operating conditions and load variations. Generally, heat pumps have an auxiliary heat generator for use when the outside temperatures become very low, for example below freezing temperatures. Such auxiliary heaters are often electrical resistance heaters or hot water coils. The heat pump controllers are called upon to operate both the primary heat pump system for heating and cooling and also the backup or auxiliary heaters according to the ambient temperature conditions and load requirements determined by the desired temperature of the environment being heated or cooled.
Several devices are available which have the ability to operate the auxiliary heaters on heat pump systems. Of course simple thermostats have long been available, and several multistage thermostats are also available. Multistage thermostats are intended to operate the primary heat pump system, but when additional heating is required the auxiliary heater is also actuated.
In some cases, an adjustable outdoor thermostat is used to operate the auxiliary heater during cold weather, but adjustment problems on such devices lead to problems. For example, if the device is set too high, the auxiliary heat may engage prematurely causing high energy consumption, and if it is too low, the result would be excessive operating time with little indoor heating provided.
Moreover, differences exist between different heat pumps systems as to when the auxiliary heat is required.
The most common heat pump control is a multistage thermostat, which activates the auxiliary heat with a secondary temperature control in the thermostat itself. This secondary switch is typically set 3.degree. F. colder that the primary switch. Thus, if the indoor temperature is more than 3.degree. F. lower than the desired temperature, the auxiliary heat is engaged. The problem with such systems is that once there is a 3.degree. F. decrease in temperature, the environment becomes uncomfortable and the primary system is forced to run constantly. In addition, the auxiliary heat is not permitted to satisfy the thermostat but is disengaged one or two degrees short of the desired room temperature. Therefore the primary system operates continuously and the auxiliary stage cycles intermittently, resulting in excessive energy consumption. These conditions, in part, give rise to the sensation of "cool heat" common to heat pump systems.
A more recent development in heat pump control devices has been a device which includes two sensors, one in the supply side of the indoor coil and one located outside. The indoor sensor measures air temperature leaving the indoor coil and if it doesn't meet the pre-set temperature requirement for that sensor, the auxiliary heater is turned on. Generally the setting is at about 96.degree. F. The purpose of the outdoor sensor is to make certain that the auxiliary heat is not engaged in the cooling mode. One problem with this system is that the lower the thermostat setting, the greater is the likelihood that air leaving the indoor coil will not reach the 96.degree. F. setting, thus engaging the auxiliary heat more often. Further, the lack of a time delay in such systems means that the auxiliary heat will operate automatically at start-up because it is unlikely that the air will be at 96.degree. F. at the beginning of the cycle.
Such conditions indicate that the auxiliary heat would operate more often and for longer periods of time resulting in higher energy consumption and higher cost.
Another type of control system is disclosed in U.S. Pat. No. 5,367,601 to Hannabery. This system utilizes a sensor in the ductwork downstream of the heating elements and compares the temperature in the ductwork to a setpoint for the ductwork temperature, and controls the duct temperature setpoint in relation to the temperature in the heated space.
Other control devices are shown in U.S. Pat. No. 4,381,814 to Funk, U.S. Pat. No. 4,387,763 to Benton and U.S. Pat. No. 4,716,957 to Thompson et al and U.S. Pat. No. 3,768,545 to Wills.
The primary object of the present invention is to provide a control device for heat pumps which overcomes the disadvantages of prior art control systems.
Another object of the invention is to provide a heat pump control device which enables maximum energy efficient use of the auxiliary heater.
Still another object of the invention is to provide a heat pump control which prevents operation of the auxiliary heater when the heat pump is operating in the cooling or "air conditioning" mode.
These and other objects and advantages of the present invention will become apparent from a detailed consideration of the present application and the accompanying claims, when considered with the accompanying drawings.